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Bacterial Physiology (Micr430)
Lecture 17
Bacterial Responses to
Environmental Stress
(Text Chapter: 19)
Environmental Stress and
Response
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Bacteria respond in a variety of
physiological ways to environmental
stresses such as changes in pH,
temperature, food source, radiation
damages and oxidative toxicity.
We will describe:
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Heat-shock response
DNA damage response
SOS response
Thermal Stress and Heat Shock
Response
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Upon a shift from 30º to 42º C, E. coli and
other bacteria transiently increase the rate of
synthesis of a set of proteins called heat
shock proteins (HSPs).
Many of these HSPs are required for cell
growth or survival at more elevated
temperatures (thermal-tolerance).
Heat shock proteins are classified based on
molecular weights
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Hsp70 family (DnaK)
Hsp60 family (GroEL)
Hsp40 family (DnaJ)
Heat Shock Response
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In E. coli, after a temperature upshift (32 ºC
to 42 ºC), there is a transient increase in the
amount of sigma factor σ32, also called σH or
RpoH.
σ32 recognizes promoters of genes in a major
heat shock regulon – the σ32 regulon.
During growth at 30 ºC, σ32 can be degraded
by several proteases. However, if σ32 is bound
to RNAP, it is protected from degradation.
Heat Shock Response
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Cis-acting mRNA sites within the 5’
region of rpoH (σ32 or σH ) message
form temp sensitive secondary
structures that sequester the ribosomal
binding site.
At higher temperature, these secondary
structures melt, thereby enabling more
efficient translation of the rpoH
message.
Heat Shock Response
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The cell uses the DnaK-DnaJ-GrpE chaperon
team to interact with σ32 at low temperature,
sequestering σ32 from RNAP.
Failure to bind RNAP facilitates degradation of
the σ32 factor.
Upon heat shock, there is an increase in the
number of other unfolded or denatured
proteins that can bind to DnaK or DnaJ. This
reduces the level of free DnaK/DnaJ
molecules available to bind σ32, allowing σ32
to bind RNAP, which protects it from
degradation.
(From Moat book)
Functions of E. coli Hsps
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Fold newly synthesized proteins at
all temperatures
Export proteins at all temperatures
Refold misfolded polypeptides
Perform proteolysis of improperly
folded or otherwise abnormal
proteins
DNA Damage Repair
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UV-damaged DNA: by
photoreactivation
Damaged DNA: by nucleotide
excision repair
Damaged DNA: by base excision
repair
Repairing UV-damaged DNA
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Exposure to UV can cause
formation of thymine dimers
During photoreactivation, a special
enzyme called DNA photolyase
reverses the dimerization reaction
upon absorbing blue light.
Nucleotide Excision Repair
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In E. coli, the products of three genes,
uvrA, uvrB and uvrC, encode UvrABC
endonuclease
It recognizes distortion in DNA region
caused by UV-induced thymine dimers
and cuts both sides of the dimer
Gap is filled in by DNA pol I and sealed
by DNA ligase.
Repair of Deaminated Bases
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A common type of DNA damage is the
deamination of bases (amino group is
replaced by keto group)
Deaminated bases pair with wrong bases
during replication, creating mutations
To repair, deaminated bases are removed by
DNA glycosylases – catalyzing breakage of
the N-glycosyl bond between the base and
the sugar, leaving an apyrimidinic or apurinic
site. These sites are called AP sites
Repair of Deaminated Bases
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After the base is removed, AP endonucleases
cleave the phosphodiester bond next to the
AP site, usually on the 5’ side
This generates 3’-hydroxyl end, which is
extended by DNA pol I as the 5’-exonuclease
activity of this enzyme removes a portion of
damaged DNA ahead.
DNA ligase seals the nick.
SOS response
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SOS is response to distress signal
generated by DNA damage
This results in the induction of over 20
unlinked genes whose products are
involved in repairing DNA damages,
allowing DNA replication to proceed
past the damaged site and stalling the
cell division to give the cell time to
repair damaged DNA.
SOS response
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SOS signal is generated when RecA
protein binds to ssDNA (produced
during DNA damage)
When the signal is received, the SOS
regulon is induced by inactivation of a
repressor LexA.